1. Field of the Invention
This invention relates to a composition including volatile monomers, that is useful for thermoset molding and to methods for molding such compositions.
2. Description of the Related Art
Compositions for thermoset molding, also known as bulk molding compound (BMC), sheet molding compound (SMC) or thick molding compound (TMC), are well known and widely used. The compositions can be formed into shapes by compression, transfer or injection molding. During the molding process, thermally-initiated polymerization and/or crosslinking reactions take place resulting in the compositions being polymerized or "cured" in the mold cavity shape. The resulting articles that exihibit heat resistance, strength, rigidity and dimensional precision. The types of resins that are typically used include unsaturated polyesters, allyls, aminos, epoxies, phenolics and silicones. However, few of these compositions have been to be useful in producing articles/materials that have the required aesthetics and fabricability for use in decorative surfacing applications.
Acrylics, in general, have not been widely used for thermoset molding in solid surfacing applications because of the high volatility of the acrylic monomers, particularly methylmethacrylate. When introduced to preheated molds, a portion of the monomer can volatilize before the polymerization/crosslinking reactions take place, resulting in poorer physical properties and visual defects. Furthermore, conventional molding of such compounds often result in formation of internal and external voids, also related to the high volatility of the acrylic monomers.
Acrylic molding materials including a single thermal initiator are known. For example, published Japanese applications H9-110496A, H9-110497A, H9-111084A, H9-111085A, and H9-111086A disclose compositions including methyl methacrylate monomer polymethyl methacrylate polymer and a resin polymer powder having a core-shell structure.
The molding of thermoset molding compounds is generally accomplished via three fundamental molding techniques: compression molding, transfer molding, and injection molding. A description of these molding techniques can be found in Wright, Ralph E., Molded Thermosets; A Handbook for Plastics Engineers, Molders, and Designers, Hanser Publishers, Oxford University Press, New York, 1991.
The choice of molding technique is largely determined by the design and functional requirements of the molded article and the need to produce the molded article economically. Although each of these methods bear some resemblance to one another, each has its own design and operational requirements. Factors to consider in choosing a molding technique for making an article include, for example, article design features, mold design, molding procedures, press selection and operation, and postmolding tools and fixtures.
Compression molding generally employs a vertical, hydraulically operated press which has two platens, one fixed and one moving. The mold halves are fastened to the platens. The premeasured molding compound charge is placed into the heated mold cavity, either manually or automatically. Automatic charging involves use of process controls and allows wider application of the molding process. The mold is then closed with application of the appropriate pressure and temperature. At the end of the molding cycle, the mold is opened hydraulically and the molded part is removed.
Compression molding mold design consists fundamentally of a cavity with a plunger. Depending upon final part design, the mold will have various slides, ejection pins, and/or moving plates to aid in mold operation and extraction of the molded article. The mold flash gap and dimensional tolerances can be adjusted to accommodate compound characteristics and part requirements.
Transfer molding is similar to compression molding except for the method in which the charge is introduced into the mold cavity. This technique is typically applied to multiple cavity molds. In this method, the charge is manually or automatically introduced into a cylinder connected to the mold cavities via a system of runners. A screw can be employed to introduce the material into the transfer cylinder. A secondary hydraulic unit is used to power a plunger which forces the molding compound through the runners and into the mold cavities of the closed mold. A vertical, hydraulic press then applies the needed pressure at the appropriate temperature to compression mold the intended part. Transfer mold design is somewhat more complicated than that of compression molds due to the presence of the transfer cylinder and runners and due to internal mold flow considerations, but general attributes are similar. Use of a shuttle press can be employed to allow encapsulation of molded-in inserts.
In general, injection molding is closely related to transfer molding except that the hydraulic press is generally horizontally oriented, and the molding compound is screw injected into the closed mold cavities via a sprue bushing and a system of gates and runners. Pressure is then applied at the appropriate temperature to cure the part. The mold is opened for part ejection and removal, the mold is closed, and the next charge is injected by the screw. This thermoset molding technique has a significant advantage in cycle time versus the other techniques listed above. As such, it finds widespread use in multicavity molding applications. Injection mold designs are yet more complex and require special attention to internal mold flow of the molding compound. In an extended application of injection molding, a vertically oriented shuttle press can be employed to allow encapsulation of molded-in inserts.
In summary, the compression molding technique is primarily a semiautomatic method which typically exhibits the least part shrinkage and the highest part density, but has the longest cycle time, is limited in ability to produce molded-in inserts, is limited in complexity of mold design, and requires the most work to finish the molded product (flash removal). Transfer molding and injection molding are semiautomatic and automatic methods, respectively, with shorter method cycle times, excellent operability in producing molded-in inserts, and less work in finishing molded parts. Both techniquess typically exhibit a lower part density and increased shrinkage versus compression molding.
Despite process differences in the molding techniques, thermoset molds have several common features in their design and use. These molds are often run isothermally; an optimal molding temperature is maintained throughout the molding cycle. For cycle time reasons, temperature cycling of molds is not common in high productivity applications. High productivity molds are designed with internal channels for circulating hot oil or with internal electric heating elements for faster mold heating response. If needed, cooling channels (oil or water) can be included. Molds can also be heated and cooled by contact with heated/cooled platens; this is representative of long cycle, low volume production. Finally, typical thermoset molding cycles involve immediate application of the final molding pressure although pressure profiles (gradual application of pressure to a final selected molding pressure later in cure cycle) are used in various situations.